package com.zhoushihao.class0806;

import java.util.Scanner;

public class PointCount {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        int count = 0;
        for (int i = 1; i < n-1; i++) {
            int tmp = (arr[i]-arr[i-1])*(arr[i]-arr[i+1]);
            if(tmp>0)
                count++;
        }
        System.out.println(count);
    }
}
